Method of positioning a sensor for determining blood pressure of an artery

ABSTRACT

The present invention is a method for locating a sensor over an underlying artery having a blood pulse. The sensor is positioned at a plurality of locations above a known appoximate location of the artery while applying a constant hold down pressure to the artery. The sensor is finally positioned at the location which exhibits the largest maximum pressure ampltiude.

This application is a continuation of Ser. No. 08/389,826, filed Feb.16, 1995, now U.S. Pat. No. 5,832,924.

The present invention relates to systems for measuring arterial bloodpressure. In particular, the invention relates to a method and apparatusfor measuring arterial blood pressure in a relatively continuous andnon-invasive manner.

Blood pressure has been typically measured by one of four basic methods:invasive, oscillometric, auscultatory and tonometric. The invasivemethod, otherwise known as an arterial line (A-Line), involves insertionof a needle into the artery. A transducer connected by a fluid column isused to determine exact arterial pressure. With proper instrumentation,systolic, mean and diastolic pressure may be determined. This method isdifficult to set up, is expensive and involves medical risks. Set up ofthe invasive or A-line method poses problems. Resonance often occurs andcauses significant errors. Also, if a blood clot forms on the end of thecatheter, or the end of the catheter is located against the arterialwall, a large error may result. To eliminate or reduce these errors, theset up must be adjusted frequently. A skilled medical practitioner isrequired to insert the needle into the artery. This contributes to theexpense of this method. Medical complications are also possible, such asinfection or nerve damage.

The other methods of measuring blood pressure are non-invasive. Theoscillometric method measures the amplitude of pressure oscillations inan inflated cuff. The cuff is placed against a cooperating artery of thepatient and thereafter pressurized or inflated to a predeterminedamount. The cuff is then deflated slowly and the pressure within thecuff is continually monitored. As the cuff is deflated, the pressurewithin the cuff exhibits a pressure versus time waveform. The waveformcan be separated into two components, a decaying component and anoscillating component. The decaying component represents the mean of thecuff pressure while the oscillating component represents the cardiaccycle. The oscillating component is in the form of an envelope startingat zero when the cuff is inflated to a level beyond the patient'ssystolic blood pressure and then increasing to a peak value where themean pressure of the cuff is equal to the patient's mean blood pressure.Once the envelope increases to a peak value, the envelope then decays asthe cuff pressure continues to decrease.

Systolic blood pressure, mean blood pressure and diastolic bloodpressure values can be obtained from the data obtained by monitoring thepressure within the cuff while the cuff is slowly deflated. The meanblood pressure value is the pressure on the decaying mean of the cuffpressure that corresponds in time to the peak of the envelope. Systolicblood pressure is generally estimated as the pressure on the decayingmean of the cuff prior to the peak of the envelope that corresponds intime to where the amplitude of the envelope is equal to a ratio of thepeak amplitude. Generally, systolic blood pressure is the pressure onthe decaying mean of the cuff prior to the peak of the envelope wherethe amplitude of the envelope is 0.57 to 0.45 of the peak amplitude.Similarly, diastolic blood pressure is the pressure on the decaying meanof the cuff after the peak of the envelope that corresponds in time towhere the amplitude of the envelope is equal to a ratio of the peakamplitude. Generally, diastolic blood pressure is conventionallyestimated as the pressure on the decaying mean of the cuff after thepeak where the amplitude of the envelope is equal to 0.82 to 0.74 of thepeak amplitude.

The auscultatory method also involves inflation of a cuff placed arounda cooperating artery of the patient. Upon inflation of the cuff, thecuff is permitted to deflate. Systolic pressure is indicated whenKorotkoff sounds begin to occur as the cuff is deflated. Diastolicpressure is indicated when the Korotkoff sounds become muffled ordisappear. The auscultatory method can only be used to determinesystolic and diastolic pressures.

Because both the oscillometric and the auscultatory methods requireinflation of a cuff, performing frequent measurements is difficult. Thefrequency of measurement is limited by the time required to comfortablyinflate the cuff and the time required to deflate the cuff asmeasurements are made. Because the cuff is inflated around a relativelylarge area surrounding the artery, inflation and deflation of the cuffis uncomfortable to the patient. As a result, the oscillometric and theauscultatory methods are not suitable for long periods of repetitiveuse.

Both the oscillometric and auscultatory methods lack accuracy andconsistency for determining systolic and diastolic pressure values. Theoscillometric method applies an arbitrary ratio to determine systolicand diastolic pressure values. As a result, the oscillometric methoddoes not produce blood pressure values that agree with the more directand generally more accurate blood pressure values obtained from theA-line method. Furthermore, because the signal from the cuff is very lowcompared to the mean pressure of the cuff, a small amount of noise cancause a large change in results and result in inaccurate measured bloodpressure values. Similarly, the auscultatory method requires a judgmentto be made as to when the Korotkoff sounds start and when they stop.This detection is made when the Korotkoff sound is at its very lowest.As a result, the auscultatory method is subject to inaccuracies due tolow signal-to-noise ratio.

The fourth method used to determine arterial blood pressure has beentonometry. The tonometric method typically involves a transducerincluding an array of pressure sensitive elements positioned over asuperficial artery. Hold down forces are applied to the transducer so asto flatten the wall of the underlying artery without occluding theartery. The pressure sensitive elements in the array typically have atleast one dimension smaller than the lumen of the underlying artery inwhich blood pressure is measured. The transducer is positioned such thatat least one of the individual pressure sensitive elements is over atleast a portion of the underlying artery. The output from one of thepressure sensitive elements is selected for monitoring blood pressure.The pressure measured by the selected pressure sensitive element isdependent upon the hold down pressure used to press the transduceragainst the skin of the patient. These tonometric systems measure areference pressure directly from the wrist and correlate this witharterial pressure. However, because the ratio of pressure outside theartery to the pressure inside the artery, known as gain, must be knownand constant, tonometric systems are not reliable. Furthermore, if apatient moves, recalibration of the tonometric system is requiredbecause the system may experience a change in gains. Because theaccuracy of these tonometric systems depends upon the accuratepositioning of the individual pressure sensitive element over theunderlying artery, placement of the transducer is critical.Consequently, placement of the transducer with these tonometric systemsis time-consuming and prone to error.

The oscillometric, auscultatory and tonometric methods measure anddetect blood pressure by sensing force or displacement caused by bloodpressure pulses as the underlying artery is compressed or flattened. Theblood pressure is sensed by measuring forces exerted by blood pressurepulses in a direction perpendicular to the underlying artery. However,with these methods, the blood pressure pulse also exerts forces parallelto the underlying artery as the blood pressure pulses cross the edges ofthe sensor which is pressed against the skin overlying the underlyingartery of the patient. In particular, with the oscillometric and theauscultatory methods, parallel forces are exerted on the edges or sidesof the cuff. With the tonometric method, parallel forces are exerted onthe edges of the transducer. These parallel forces exerted upon thesensor by the blood pressure pulses create a pressure gradient acrossthe pressure sensitive elements. This uneven pressure gradient createsat least two different pressures, one pressure at the edge of thepressure sensitive element and a second pressure directly beneath thepressure sensitive element. As a result, the oscillometric, auscultatoryand tonometric methods produce inaccurate and inconsistent bloodpressure measurements.

SUMMARY OF THE INVENTION

The present invention is a method for locating a sensor over anunderlying artery having a blood pulse. The sensor is positioned at aplurality of locations above a known appoximate location of the arterywhile applying a constant hold down pressure to the artery. The sensoris finally positioned at the location which exhibits the largest maximumpressure ampltiude. This method more accurately locates the sensor overthe underlying artery so that more accurate blood pressure measurementsmay be taken.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a blood pressure monitoring systemhaving a sensor assembly mounted upon the wrist of a patient.

FIG. 2 is a side view of the wrist assembly of the blood pressuremonitoring system of FIG. 1.

FIG. 3 is an end view of the wrist assembly.

FIG. 4 is a cross-sectional view of the wrist assembly.

FIG. 4A is an expanded cross-sectional view of the sensor interfacealong section 4A--4A of FIG. 4.

FIG. 5 is a top view of the wrist assembly and cylinder of the system ofFIG. 1.

FIG. 6 is a bottom view of the wrist assembly and cylinder with aportion removed.

FIG. 7 is an electrical block diagram of the blood pressure monitoringsystem of FIG. 1.

FIG. 8 is a front elevational view of a monitor of the blood pressuremonitoring system of FIG. 1.

FIG. 9 is a graph illustrating blood pressure waveforms.

FIG. 10 is a graph illustrating a curve fit from points taken from thewaveforms of FIG. 9.

FIG. 11 is a graph illustrating a corrected and scaled waveform takenfrom the waveforms of FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS I. Overview

FIG. 1 illustrates blood pressure monitoring system 20 for measuring anddisplaying blood pressure within an underlying artery within wrist 22 ofa patient. Monitoring system 20 includes wrist assembly 24, monitor 26,cylinder 28, cable 30 and cable 32.

Wrist assembly 24 is mounted on wrist 22 for applying a varying holddown pressure to an artery within wrist, and for sensing blood pressurewaveforms produced in the artery. Wrist assembly 24 includes swivelmount 34, hold down assembly 36, sensor interface assembly 38, waveformpressure transducer 40, hold down pressure transducer 42, connectiontube 44, wrist mount 46 and wrist pad 48.

Cylinder 28, under the control of monitor 26, supplies fluid pressurethrough cable 32 to wrist assembly 24 to produce the varying hold downpressure. Cylinder 28 includes a movable piston which is driven by astepper motor or linear actuator.

Electrical energization to wrist assembly 24 and pressure waveformsensor signals to monitor 26 are supplied over electrical conductorsextending between monitor 26 and wrist assembly through cable 30,cylinder 28 and cable 32. Drive signals to cylinder 28 are supplied frommonitor 26 through electrical conductors within cable 30.

Monitor 26 receives the pressure waveform sensor signals from wristassembly 24, digitizes the signals to produce pressure waveform data fora plurality of beats, and performs waveform analysis on the data. Thewaveform analysis extracts a plurality of waveform parameters, whichpreferably include waveform shape, relative amplitude and gainparameters. From the waveform parameters, monitor 26 calculates orotherwise derives blood pressure values, such as mean blood pressure,diastolic blood pressure and systolic blood pressure. Monitor 26 thendisplays the derived blood pressure values.

As shown in FIG. 1, monitor 26 includes control switches or input keys50a-50g, digital displays 52a-52c and display screen 54. Input keys50a-50c comprise hard keys for controlling monitor 26. Input keys50d-50g consist of software programmable keys which are adaptable forvarious functions. Digital displays 52a-52c continually displaysystolic, diastolic and mean blood pressure, respectively. Displayscreen 54 displays the blood pressure pulse waveforms and prompts toguide the operator.

In operation, sensor interface assembly 38 is positioned over the radialartery. Wrist mount 46 maintains the position of wrist assembly 24including sensor interface assembly 38 on wrist 22. In response to fluidpressure supplied from cylinder 28 through cable 32, hold down assembly36 applies force and moves sensor interface assembly 38 to vary thepressure applied to wrist 22 above the radial artery.

As this pressure is varied, distinct arterial pressure waveforms areexhibited by the blood pressure pulse within the underlying artery. Eachwaveform corresponds to a cardiac cycle. Each arterial pressure waveformor shape is obtained by sensing and measuring pressures exhibited by thepulse of the underlying artery versus time during an individual cardiaccycle. Arterial pressure applied to sensor interface assembly 38 and istransferred as a fluid pressure from interface assembly 38 to waveformpressure transducer 40 through tube 44. The electrical sensor signalsfrom transducer 40 are supplied to monitor 26 for digitization andanalysis.

The amplitude of each sensed waveform is a function of the appliedpressure applied to the artery by sensor interface assembly 38 and theamplitude of the arterial pulse. The shape characteristics of at leastone waveform and other parameters derived from the sensed waveforms areused by digital signal processing circuitry of monitor 26 to determinesystolic, mean and diastolic pressure. The calculated pressures aredisplayed by displays 52a-52c and display screen 54.

II. Wrist Assembly 24

Wrist assembly 24 is shown in further detail in FIGS. 2-6. Swivel mount34 and hold down assembly 36 are side-by-side, and are pivotallyconnected by swivel joint 60. Swivel mount 34 carries transducers 40 and42 and wrist pad 48. Sensor interface assembly 38 is pivotally connectedto and is positioned below hold down assembly 36. Wrist mount 46, whichincludes flexible wrist band 62 and wire loops 64 and 66, is connectedbetween an outer end of swivel mount 34 and teeter mount 68 at anopposite outer end of hold down assembly 36.

FIG. 2 is a side elevational view illustrating wrist assembly 24 ingreater detail. Swivel mount 34 is a U-shaped body. Swivel joint 60 isformed by a socket 70 of swivel mount 34 and swivel ball 72 of hold downassembly 36. Socket 70 extends into a channel within the U-shapedconfiguration of swivel mount 34 and is sized for receiving swivel ball72 which projects from an inner end wall of hold down assembly 36. Theball socket swivel joint provided by ball 72 and socket 70 permit swivelmount 34 and hold down assembly 36 to rotate and pivot in virtually anydirection so as to better conform to wrist 22. To aid in pivoting swivelmount 34 with respect to hold down assembly 36, swivel mount 34 includesan arcuate or beveled lower edge 74 along its inner end. Beveled edge 74permits hold down assembly 36 to pivot downward so as to wrap aroundwrist 22 (or alternate anatomy) of a patient.

Swivel mount 34 further includes a tightening screw 76 which extendsacross swivel mount 34 adjacent socket 70 and ball 72. Tightening screw76 permits socket 70 of swivel mount 34 to be tightened about ball 72 soas to increase friction between socket 70 and ball 72 to adjust thelevel of force necessary to readjust the positioning of swivel mount 34and hold down assembly 36. Untightening screw 76 permits ball 72 to bereleased from socket 70 such that hold down assembly 36 and sensorinterface assembly 38 may be disassembled from swivel mount 34.

FIG. 3 is a end elevational view of blood pressure monitoring system 20of FIG. 1, illustrating teeter mount 68 in greater detail. As shown byFIG. 3, teeter mount 68 includes fulcrum 80 and tightening screw 82.Fulcrum 80 is generally a triangular shaped member having two opposingslanted top surfaces. Fulcrum 80 is coupled to loop 66 and thereby towrist band 62. Fulcrum 80 teeters about hold down assembly 36 andpermits loop 66 and wrist band 62 to be adjustably positioned so as tobetter conform to wrist 22. Tightening screw 82 extends through fulcrum80 and threadably engages hold down assembly 36. Tightening screw 82tightens fulcrum 80 against hold down assembly 36 so that the positionof fulcrum 80 may be frictionally set. In FIG. 3, fulcrum 80 is shown ina middle position, and can be rotated either a clockwise orcounterclockwise direction as needed.

Wrist assembly 24 stably and securely positions sensor interfaceassembly 38 over the underlying artery of the patient. Swivel mount 34may be rotated and pivoted in practically all directions about socket 70and ball 72. Furthermore, teeter mount 68 permits wrist band 62 to beteetered or adjusted so as to better conform with wrist 22 of thepatient. Wrist band 62 wraps around wrist 22 to secure sensor interfaceassembly 38 and wrist pad 48 adjacent wrist 22 of the patient. Becausesensor interface assembly 38 is more securely and stably positionedabove the underlying artery of wrist 22, patient movement is less likelyto reposition sensor interface assembly 38. As a result, sensorinterface assembly 38 can be reliably located over the underlying arteryso that more accurate and consistent blood pressure measurements may betaken.

As shown in FIG. 4, swivel mount 34 carries waveform pressure transducer40, hold down pressure transducer 42, and wrist pad 48. Waveformpressure transducer 40 senses blood pressure waveforms from the arterywhich is transmitted to transducer 40 from sensor interface assembly 38through fluid tube 44 (FIG. 1). Hold down pressure transducer 42 sensesfluid pressure supplied by cylinder 28 to hold down assembly 36, and isused as a safety feature to detect an excess hold down pressurecondition. Wrist pad 48 is preferably adhesively secured to plate 90 ata bottom surface of swivel mount 34. Pad 48 is preferably made of a softflexible and compressible material so that swivel mount 34 betterconforms to the wrist of a patient. Plate 90 is preferably made of ametal such as brass and is screwed to swivel mount 34 by screw 92.Conductive plate 94 is secured within swivel mount 34 and is spaced fromplate 90 so that transducer 40 is positioned between plates 90 and 94.Transducer 40 preferably has a metallic conductive surface such as brasswhich contacts conductive plate 94, which is electrically grounded. As aresult, brass plate 94 electrically grounds transducer 40 so as to drainstatic charge from transducer 40.

As shown by FIG. 4, hold down pressure assembly 36 includes swivel ball72, housing 100, diaphragm 102, ring 104, piston 106, piston rod 108,pin 110 and pin mount 112. Diaphragm 102 comprises a generally circularsheet of flexible material such as reinforced rubber. Diaphragm 102 isspaced from and cooperates with interior cavity 114 formed withinhousing 100 to define pressure chamber 116. Pressure chamber 116 extendsgenerally above and partially around piston 106. Pressure chamber 116receives pressurized fluid from cylinder 28 through fluid passage 118such that diaphragm 102 expands and contracts to drive piston 106 andpiston rod 108 up and down. As a result, a selected pressure may beapplied to piston 106 and piston rod 108 so as to selectively apply apressure to sensor interface assembly 38, which is pivotally mounted tothe lower end of piston rod 108. By varying the volume of fluid withinpressure chamber 116, blood pressure monitoring system 20 applies avarying hold down pressure to sensor interface assembly 38 and theunderlying artery.

Diaphragm 102 is supported in place by ring 104. Ring 104 encircles theouter perimeter of diaphragm 102 and captures an outer perimeter or edgeportion of diaphragm 102 between ring 104 and housing 100 so as to sealdiaphragm 102 against housing 100. Ring 104 is preferably adhesivelysecured to housing 100 and diaphragm 102.

Piston 106 is preferably a disk or cylinder shaped member which has itstop surface preferably fixedly coupled (such as by an adhesive) todiaphragm 102. Consequently, as fluid is supplied to chamber 116, thevolume of chamber 116 expands by moving piston 106 downward. Bore 120extends from top to bottom of piston 106 and is sized for receiving aportion of piston rod 108. Piston 106 mates with piston rod 108 andexerts pressure upon piston rod 108 and sensor interface assembly 38.

Piston rod 108 is coupled to piston 106 and sensor interface assembly38. Piston rod 108 includes plug 122, flange 124, stem 126, ball 128 andpin hole 130. Plug 122 is cylindrically shaped and is press fit withinbore 120 to secure piston rod 108 to piston 106. Flange 124 projectsoutwardly from plug 122 and fits within a depression formed in thebottom surface of piston 106. As a result, piston 106 presses againstflange 124 of piston rod 108 to drive piston rod 108. Alternatively,because piston rod 108 is secured to piston 106 by plug 122, piston 106lifts piston rod 108 as pressure is decreased within pressure chamber116. Stem 126 integrally extends downward from flange 124 and has alength extending into interface assembly 38. Ball 128 is integrallyformed at the lower end of stem 126 and is received within socket 132 ofsensor interface assembly 38. As a result, sensor interface assembly 38pivots about ball 128 of piston rod 108.

Pin hole 130 axially extends through piston rod 108 and is sized forreceiving pin 110. Pin 110 is fixedly secured to housing 100 by pinmount 112 and extends through housing 100 into pin hole 130. Pin 110 hasa diameter smaller than the diameter of pin hole 130 and extends intostem 126. Pin 110 guides the up and down movement of piston 106 andpiston rod 108 as pressure within pressure chamber 116 is varied. Pin110 prevents lateral movement of piston 106 and piston rod 108 so thatpiston 106 and piston rod 108 apply only a perpendicular force to sensorinterface assembly 38. As a result, pin 110 permits piston 106 andpiston rod 108 to move up and down while pin 110 remains fixedlysupported by pin mount cap 112 to housing 100. Pin 110 is preferablymade from a hard rigid material such as stainless steel.

As shown by FIG. 4, hold down pressure assembly 28 further includespressure supply passage 118, which extends from pressure chamber 116through swivel ball 72 where it connects with flexible tubes 140 and 142(shown in FIGS. 5 and 6). Flexible tube 140 extends through cable 32from cylinder 28 to passage 118 in swivel ball 72. Flexible tube 142connects passage 118 to transducer 42 in swivel mount 34. This allowstransducer to monitor the fluid pressure in chamber 116. Fluid supplytube 140 applies pressurized fluid from cylinder 28 into pressurechamber 116 to vary the pressure within chamber 116 so as to drivepiston 106 and piston rod 108.

FIGS. 4 and 4A illustrate sensor interface assembly 38 in detail. FIG. 4is a cross-sectional view of wrist assembly 24. FIG. 4A is an enlargedcross-sectional view of sensor interface assembly 38, taken alongsection 4A-4A of FIG. 4. Sensor interface assembly 38 includes top plate150, upper V mount 152, lower V mount 154, diaphragm lock 156, innermounting ring 158, outer mounting ring 160, side wall diaphragm 162,damping ring 164, inner diaphragm 166 and outer diaphragm 168.

Top plate 150 is a generally flat annular platform having a central bore200, shoulder 202, shoulder 204, and side bore 206. Central bore 200receives and holds lower V mount 154. Upper V mount 152 engages shoulder202 and extends downward into bore 200 and into lower V mount 154. Rings158 and 160 and the upper outer end of side wall diaphragm 162 aremounted in shoulder 204.

Side bore 206 is defined within top 150 and extends through top 150 soas to be in communication with fluid passage 208 defined between upperand lower V mounts 152 and 154 and between upper V mount 152 anddiaphragm lock 156. Side bore 206 receives an end of tube 44 so thattube 44 is in fluid communication with fluid passage 208 and sensorinterface chamber 210 (which is defined by diaphragms 166 and 168).Fluid passage 208 and tube 44 provide fluid communication between sensorinterface chamber 210 and transducer 40 eccentric to socket 132. As aresult, piston rod 108 may be pivotally connected to sensor interfaceassembly 38 at a lower pivot point.

Upper V mount 152 is a funnel-shaped socket which is sized for receivingthe lower or distal end of piston rod 108. Preferably, upper V mount 152extends through central bore 200 of top plate 150 to a location nearsensor interface chamber 210. Upper V mount 152 is fixedly secured to anupper portion of top plate at shoulder 202. Upper V mount 152 issupported by top plate 150 such that upper V mount 152 is spaced fromlower V mount 154 to define annular fluid passage 208. Fluid passage 208is in fluid communication with sensor interface chamber 210. A fluidcoupling medium fills chamber 210, passage 208 and tube 44 all the wayto transducer 40. Upper V mount 152, which is made from a material suchas nylon and forms detent 220 and socket 132 for pivotally receiving aball member 128 of piston rod 108. As a result, sensor interfaceassembly 38 may be pivoted about socket 132 so as to better conform tothe anatomy of the patient. Furthermore, because socket 132 is adjacentto sensor interface chamber 210, sensor interface assembly 38 ispivotally coupled to piston rod 108 about a low pivot point This permitssensor interface assembly 38 to be stably positioned above theunderlying artery. In addition, the low pivot point enables hold downassembly 36 to apply a more direct, uniform force on diaphragm 168.Thus, the hold down pressure applied by hold down pressure assembly 36is more uniformly applied to the anatomy of the patient above theunderlying artery.

Lower V mount 154 is a generally cylindrical shaped member includingstep or spar 230 and bore 232. An outer surface or perimeter of lower Vmount 154 projects outwardly to form spar 230. Spar 230 engages thelower surface of top plate 150 to partially support side wall diaphragm162 which is partially captured between top plate 150 and spar 230. Inthe preferred embodiment, adhesive is used between the lower surface oftop plate 150 and spar 230 to fixedly secure the portion of side walldiaphragm 162 trapped therebetween. Alternatively, spar 230 may be pressfit against the lower surface of top plate 150 to secure and supportside wall diaphragm 162. Spar 230 further divides the outer perimeter oflower V mount 154 into two portions, an upper portion 234 and a lowerportion 236. Upper portion 234 fits within bore 200 of top plate 150.Upper portion 234 is preferably adhesively secured to top plate 150within bore 200. Lower portion 236 extends below spar 230. Lower portion236, spar 230 and side wall diaphragm 162 define expansion cavity 240.Expansion cavity 240 enables upper diaphragm 166 to initially changeshape while only experiencing a small change in volume.

Diaphragm lock 156 is a thin, elongated, annular ring including bore 250and lower lip 252. Bore 250 extends through diaphragm lock 156 and withupper V mount 152, defines a portion of fluid passage 208. Lip 252projects outwardly from a lower end of diaphragm lock 156. Diaphragmlock 156 fits within bore 232 of lower V mount 154 until an inner edgeof diaphragm lock 156 is captured between inserts, lip 252 and the lowerend of lower V mount 154. Diaphragm lock 156 is preferably adhesivelyaffixed to lower V mount 154. Alternatively, diaphragm lock 156 may bepress fit within lower V mount 154.

Side wall diaphragm 162, rings 158 and 160 and top plate 150 define anannular deformable chamber 260 coupled between top plate 150 and ring164. Side wall diaphragm 162 is preferably formed from a generallycircular sheet of flexible material, such as vinyl, and is partiallyfilled with fluid. Diaphragm 162 has a hole sized to fit around upperportion 234 of lower V mount 154. Diaphragm 162 includes outer edgeportion 162a and inner edge portion 162b. Outer edge portion 162a istrapped and held between outer ring 160 and top plate 150. Inner edgeportion 162b is trapped and supported between top plate 150 and spar 230of lower V mount 154. Diaphragm 162 is made from a flexible material andis bulged outward when chamber 260 is partially filled with fluid.Chamber 260 is compressible and expandable in the vertical direction soas to be able to conform to the anatomy of the patient surrounding theunderlying artery. As a result, the distance between top plate 150 andthe patient's anatomy can vary around the periphery of side walldiaphragm 162 according to the contour of the patient's anatomy.Furthermore, because fluid is permitted to flow through and aroundchamber 260, pressure is equalized around the patient's anatomy.

Damping ring 164 generally consists of an annular compressible ring andis preferably formed from a foam rubber or other pulse dampeningmaterial such as open celled foam or closed cell foam. Ring 164 iscentered about and positioned between side wall diaphragm 162 anddiaphragms 166 and 168. Damping ring 164 is isolated from the fluidcoupling medium within chamber 210. Because ring 164 is formed from acompressible material, ring 164 absorbs and dampens forces in adirection parallel to the underlying artery which are exerted by theblood pressure pulses on sensor interface assembly 38 as the bloodpressure pulse crosses sensor interface assembly 38. Because bottom ring164 is isolated from the fluid coupling medium, the forces absorbed orreceived by ring 164 cannot be transmitted to the fluid coupling medium.Instead, these forces are transmitted across ring 164 and side walldiaphragm 162 to top plate 150. Because this path is distinct andseparate from the fluid coupling medium, chamber 210 and the fluidcoupling medium are isolated from these forces. In addition, ring 164also presses tissue surrounding the artery to neutralize or offsetforces exerted by the tissue.

Upper diaphragm 166 is an annular sheet of flexible material having aninner portion 166a, an intermediate portion 166b, an outer portion 166cand an inner diameter sized to fit around diaphragm lock 156. Innerportion 166a is trapped or captured between lip 252 of diaphragm lock156 and the bottom rim of lower V mount 154. Inner portion 166A ispreferably adhesively affixed between lip 252 and lower V mount 154.

Intermediate portion 166b lies between inner portion 166a and outerportion 166c. Intermediate portion 166b is adjacent to expansion cavity240 and is isolated from ring 164 and chamber 260. Because intermediateportion 166b is positioned adjacent to expansion cavity 240,intermediate portion 166b is permitted to initially move upward intoexpansion cavity 240 as chamber 260, ring 164 and outer diaphragm 168conform to the anatomy of the patient surrounding the underlying arterywhile the experiences only a small change in volume. As ring 164 ispressed against the anatomy of the patient surrounding the artery toneutralize or offset forces exerted by the tissue, diaphragm 168 is alsocompressed. However, because intermediate portion 166b is permitted toroll into expansion cavity 240, chamber 210 does not experience a largevolume decrease and a large corresponding pressure increase. Thus,sensor interface assembly 38 permits greater force to be applied to theanatomy of the patient through ring 164 to neutralize tissue surroundingthe artery without causing a corresponding large change in pressurewithin chamber 210 as the height of the side wall changes. As a result,sensor interface assembly 38 achieves more consistent and accurate bloodpressure measurements.

Outer diaphragm 168 is a generally circular sheet of flexible materialcapable of transmitting forces from an outer surface to fluid withinchamber 210. Outer diaphragm 168 is coupled to inner diaphragm 166 andis configured for being positioned over the anatomy of the patient abovethe underlying artery. Outer diaphragm sheet 168 includes non-activeportion or skirt 168a and active portion 168b. Skirt 168a constitutesthe area of diaphragm 168 where inner diaphragm 166, namely outerportion 166c, is bonded to outer diaphragm 168. Skirt 168a and outerportion 166c are generally two bonded sheets of flexible material,forces parallel to the underlying artery are transmitted across skirt168a and outer portion 166c and are dampened by the compressiblematerial of ring 164.

Active portion 168b is constituted by the portion of outer diaphragmsheet 168 which is not bonded to inner diaphragm 166. Active portion168b is positioned below and within the inner diameter of ring 164.Active portion 168b is the active area of sensor interface assembly 38which receives and transmits pulse pressure to transducer 40. Activeportion 168b of diaphragm 168, intermediate portion 166b of diaphragm166 and diaphragm lock 156 define sensor interface chamber 210.

The coupling medium within chamber 210 may consist of any fluid (gas orliquid) capable of transmitting pressure from diaphragm 168 totransducer 40. The fluid coupling medium interfaces between activeportion 168b of diaphragm 168 and transducer 40 to transmit bloodpressure pulses to transducer 40. Because the fluid coupling medium iscontained within sensor interface chamber 210, which is isolated fromthe side wall of sensor interface assembly 38, the fluid coupling mediumdoes not transmit blood pressure pulses parallel to the underlyingartery, forces from the tissue surrounding the underlying artery andother forces absorbed by the side wall to transducer 40. As a result,sensor interface assembly 38 more accurately measures and detectsarterial blood pressure.

Sensor interface assembly 38 provides continuous external measurementsof blood pressure in an underlying artery. Because sensor interfaceassembly 38 senses blood pressure non-invasively, blood pressure ismeasured at a lower cost and without medical risks. Because sensorinterface assembly 38 is relatively small compared to the larger cuffsused with oscillometric and auscultatory methods, sensor interfaceassembly 38 applies a hold down pressure to only a relatively small areaabove the underlying artery of the patient. Consequently, blood pressuremeasurements may be taken with less discomfort to the patient. Becausesensor interface assembly 38 does not require inflation or deflation,continuous, more frequent measurements may be taken.

Furthermore, sensor interface assembly 38 better conforms to the anatomyof the patient so as to be more comfortable to the patient and so as toachieve more consistent and accurate blood pressure measurements.Because chamber 260 is deformable and partially filled with fluid,chamber 260 better conforms to the anatomy of the patient and equalizespressure applied to the patient's anatomy. Because ring 164 iscompressible and because diaphragm 168 is flexible and is permitted tobow or deform inwardly, ring 164 and diaphragm 168 also better conformto the anatomy of the patient At the same time, however, sensorinterface assembly 38 does not experience a large sudden increase inpressure in sensor interface chamber 210 as ring 164 and diaphragm 168are pressed against the anatomy of the patient. Chamber 260 and ring 164apply force to the anatomy of the patient to neutralize the forcesexerted by tissue, surrounding the underlying artery. Because chamber260 and ring 164 are both compressible, the height of the side walldecreases as side wall is pressed against the patient. Diaphragms 166and 168 are also conformable. However, because intermediate portion 166bof inner diaphragm 166 is permitted to move upward into expansion cavity240, sensor interface chamber 210 does not experience a large volumedecrease and a corresponding large pressure increase. Thus, the sidewall is able to apply a greater force to the anatomy of the patientwithout causing a corresponding large, error producing increase inpressure within sensor interface chamber 210 due to the change in heightof the side wall and the change in shape of outer diaphragm 168.

At the same time, sensor interface assembly 38 permits accurate andconsistent calculation of blood pressure. Because of the large sensingarea through which blood pressure pulses may be transmitted totransducer 40, sensor interface assembly 38 is not as dependent uponaccurate positioning of active portion 168b over the underlying artery.Thus, sensor interface assembly 38 is more tolerant to patient movementas measurements are being taken.

Moreover, sensor interface assembly 38 achieves a zero pressure gradientacross the active face or portion 168b of the sensor, achieves a zeropressure gradient between the transducer and the underlying artery,attenuates or dampens pressure pulses that are parallel to the sensingsurface of the sensor, and neutralizes forces of the tissue surroundingthe underlying artery. Sensor interface assembly 38 contacts and appliesforce to the anatomy of the patient across skirt 168a and active portion168b. However, the pressure within interface chamber 210 issubstantially equal to the pressure applied across active portion 168b.The remaining force applied by sensor interface assembly 38 across skirt168a which neutralizes or offsets forces exerted by the tissuesurrounding the underlying artery is transferred through the side wall(ring 164 and chamber 260) to top plate 150. As a result, the geometryand construction of sensor interface assembly 38 provides the properratio of pressures between skirt 168a and active portion 168b toneutralize tissue surrounding the underlying artery and to accuratelymeasure the blood pressure of the artery. In addition, because the fluidcoupling medium within sensor interface chamber 210 is isolated from theside wall, pressure pulses parallel to the underlying artery, forcesfrom tissue surrounding the underlying artery and other forces absorbedby the side wall are not transmitted through the fluid coupling mediumto transducer 40. Consequently, sensor interface assembly 38 alsoachieves a zero pressure gradient between transducer 40 and theunderlying artery.

FIG. 5 is a top view of wrist assembly 24. FIG. 5 further illustratesportions of swivel mount 34 and cable 30 in greater detail. Fluid tube140 has one end connected to passage 118 in swivel ball 72 and its otherend connected to cylinder 28.

Fluid tube 142 extends between transducer 42 and passage 118 in ball 72.Fluid tube 142 fluidly connects pressure chamber 116 and transducer 42.As a result, transducer 42 senses the pressure within pressure chamber116. Transducer 42 produces electrical signals representing the sensedhold down pressure within pressure chamber 116. These electrical signalsare transmitted by electrical wires 280 which extend within cables 30and 32 to monitor 26 (shown in FIG. 1). As a result, monitor 26 maycontinuously verify that the actual pressure within pressure chamber 116is within a safe range.

As further shown by FIG. 5, cable 32 additionally encloses electricalwires 290 from transducer 40 (shown in FIG. 4). Electrical wires 290transmit electrical signals representing blood pressure amplitudessensed by transducer 40. Cable 32 also encloses an electrical groundingwire 300 which is electrically connected through resistor 302 (FIG. 6)to brass plate 94 (shown in FIG. 4) and which electrically groundstransducers 40 and 42.

FIG. 6 is a bottom view of wrist assembly 24. FIG. 6 illustrates swivelmount 34 with pad 48 and plate 90 (FIG. 4) removed. FIG. 6 illustratesthe electrical connection between transducers 40 and 42 and electricalwires 280 and 290, respectively. As shown by FIG. 6, swivel mount 34contains electrical connector 304. Electrical connector 304 receivesleads 306 of transducer 40. Leads 306 transmit the electrical signalsproduced by transducer 40 representing the pressures and transmits theelectrical signals to electrical wires 290. Electrical connector 304further includes an electrical resistor 302 electrically coupled tobrass plate 94. Resistor 302 is further electrically coupled to groundedelectrical wire 300. As a result, static charge is drained throughresistor 302 through electrical connector 304 and through grounded wire300. Electrical connector 304 permits transducer 40 to be removed andseparated from swivel mount 34.

Similarly, transducer 42 includes four electrical leads 310 which areelectrically connected to electrical wires 280. In contrast totransducer 40, however, transducer 42 is generally fixed and mountedwithin swivel mount 34. As shown by FIG. 6, swivel mount 34 electricallyconnects transducers 40 and 42 to monitor 26 by electrical wires 280 and290 carried within cables 30 and 32.

III. Monitor 26

FIG. 7 shows a block diagram of blood pressure monitoring system 20. Asbest shown by FIG. 7, monitor 26 further includes input signal processor350, analog-to-digital converter 352, microprocessor (and associatedmemory) 354, inputs 50a-50g, cylinder drive 356, displays 52a-52c and54, and power supply 358. In operation, microprocessor 354 receivesinputted signals from inputs 50a-50g. Inputs 50a -50g may also consistof a keyboard or other input mechanisms. Inputs 50a-50g permitmicroprocessor 354 to perform a calibration.

Microprocessor 354 controls cylinder drive 356 to vary hold downpressure applied by hold down pressure assembly 36 of wrist assembly 24.Hold down pressure is applied to the anatomy of the patient directlyabove the artery. The hold down pressure applied by hold down pressureassembly 36 on sensor interface assembly 38 is increased over time. Asthe force or hold down pressure applied by sensor interface assembly 38increases, the amplitude or relative pressure of the blood pressurepulse also increases until a maximum amplitude results. Once the maximumamplitude or maximum energy transfer results, the amplitude of the bloodpressure pulse begins to decrease as the artery begins to flatten outbeyond the point of maximum energy transfer.

Transducer 40 of wrist assembly 24 senses the amplitude and shape of theblood pressure pulses within the underlying artery. Transducer 40creates electric sensor signals representing the pressures exerted bythe sensed blood pressure pulses. The sensor signals are transmitted toinput signal processor 350 of monitor 26. Input signal processor 350processes the sensor signals and filters any unwanted or undesirablenoise and other effects. The sensor signals are then transmitted frominput signal processor 350 to analog-to-digital convertor 352.Analog-to-digital convertor 352 converts the sensor signal into digitalform. A digital signal representing the pressures of the sensed bloodpressure pulses is sent to microprocessor 354.

Based upon the digital sensor signals representing the sensed pressuresand shape of the blood pressure pulses, microprocessor 354 determineswave shape information by measuring amplitude and shape versus time ofindividual cardiac cycles. The arterial wave shape information isdetermined by sampling the arterial waves at a rate significantly aboveheart rate so that a good definition of the arterial pressure wave ismeasured. From wave shape information and other parameters derivedtherefrom, microprocessor 354 calculates systolic, diastolic and meanblood pressures.

IV. Method for Locating Sensor Interface Assembly Over Artery

FIG. 8 illustrates digital displays 52a-52c and display screen 54 ofmonitor 26 in greater detail. As shown by FIG. 8, display screen 54further includes pressure scale 400, horizontal guidelines 410 anddigital readout 430. Monitor 26 also is used to display blood pressurepulse waveforms so as to guide the operator in positioning and locatingsensor interface assembly 38 directly over the underlying artery havinga blood pressure pulse so that more accurate blood pressure values maybe determined.

To place sensor interface assembly 38 over an underlying artery, sensorinterface assembly 38 is located or positioned above a known approximatelocation of the underlying artery. As sensor interface assembly 38 ispositioned over the underlying artery, a constant hold down pressure isapplied to sensor interface assembly 38 and to the underlying artery.Preferably, the pressure applied to sensor interface assembly 38 shouldbe as high as possible without the diastolic portion 440 of bloodpressure waveforms 450 distorting.

In response to the applied pressure, the underlying artery exhibits ablood pressure pulse waveform for each cardiac cycle. Sensor interfaceassembly 38 senses or receives the force exerted by the blood pressurepulse as the pulse travels beneath the sensing surface and transmits thepressures through the fluid coupling medium to transducer 40. Transducer40 in turn senses the changes in pressure and converts the pressuresinto electrical signals which represent the arterial pressure waveforms.The signals are then transmitted through cables 30 and 32 to monitor 36.Monitor 36 samples the signals preferably at a rate of 128 samples persecond. Monitor 36 then visually displays the sampled signals receivedfrom transducer 40 and displays the signals representing arterialpressure waveforms on display screen 54. Display screen 54 is preferablyindexed so as to provide a vertical scale 400 with horizontal guidelines410 for displaying pressure. Guidelines 410 permit the maximum pressureamplitude of blood pressure pulse waveforms at the particular locationand at a constant hold down pressure to be determined. A representativeseries of blood pressure pulse waveforms 450 is illustrated on screen 54in FIG. 8.

To further aid the operator in determining the maximum amplitude ofblood pressure pulse waveforms, display screen 54 further includes adigital readout 430 which digitally displays the maximum pressureamplitude exerted by the pulse in response to the hold down pressureapplied to the artery. As shown in FIG. 8, the artery exhibits pressureswhich are in the form of blood pressure pulse waveforms 450 when aconstant hold down pressure of 80 mmHg is applied to the underlyingartery. Blood pressure pulse waveforms 450 exhibit a maximum amplitudeof approximately 18 mmHg.

Once the maximum pressure amplitude exerted by the pulse at a particularhold down pressure at the particular location is determined and noted,sensor interface assembly 38 is repositioned at a second location abovethe known approximate location of the artery. The same constant holddown pressure is applied to sensor interface assembly 38 and to theunderlying artery of wrist 22. The constant hold down pressure appliedto the underlying artery is preferably as close as possible to theconstant hold down pressure applied at the first location as indicatedby display screen 54. This can be done by applying a hold down pressureto sensor interface assembly 38 at a constant force equal to one ofguidelines 410.

The maximum pressure amplitude exerted by the pulse in response to thehold down pressure applied to the artery at the second location can bedetermined from the analog display of the blood pressure waveforms 450on display screen 54 or the digital readout 430 on display screen 54.The maximum pressure amplitude at the second location is then noted orrecorded for comparison with maximum pressure amplitudes at otherlocations. Typically, sensor interface assembly 38 will be repositionedat a plurality of locations above a known approximate location of theartery while applying a constant hold down pressure to the artery. Ateach location, the maximum pressure amplitude exerted by the pulse inresponse to the constant hold down pressure will be displayed on displayscreen 54 and noted. At each location, the maximum pressure amplitudeindicated by display screen 54 is compared with maximum pressureamplitudes exerted by the pulse in response to the constant hold downpressure applied to the artery and indicated by display screen 54 at theplurality of other locations. After the maximum pressure amplitudecorresponding to each of the plurality of locations are compared, sensorinterface assembly 38 and its sensing surface are positioned at theparticular location which corresponds to the location at which thelargest of the maximum pressure amplitudes is exerted by the pulse inresponse to the constant hold down pressure applied to the artery.

V. Method for Determining Blood Pressure Values

Once the sensor is properly positioned over the underlying artery, bloodpressure monitoring system 20 determines blood pressure values from thesensed waveform pressure amplitudes sensed by sensor interface assembly38 and from other parameters derived from the pressure amplitudes usinga stored set of coefficients. A pressure amplitude is determined at eachsample point.

Blood pressure monitoring system 20 calculates a systolic blood pressurevalve (S), a mean blood pressure (M) and a diastolic blood pressure (D)based upon the following formulas:

    M=F.sub.m (P.sub.1.sup.m, . . . , P.sub.n.sup.m, C.sub.1.sup.m, . . . , C.sub.n.sup.m)

    S=F.sub.s (P.sub.1.sup.s, . . . , P.sub.n.sup.s, C.sub.1.sup.s, . . . , C.sub.n.sup.s)

    D=F.sub.d (P.sub.1.sup.d, . . . , P.sub.n.sup.d, C.sub.1.sup.d, . . . , C.sub.n.sup.d)

wherein F_(m), F_(s), F_(d) are linear or non-linear functions, P₁ ^(m),P₁ ^(s), P₁ ^(d), . . . , P_(n) ^(m), P_(n) ^(s), P_(n) ^(d) areparameters derived from waveform pressure amplitudes and C₁ ^(m), C₁^(s), C₁ ^(d), . . . , C_(n) ^(m), C_(n) ^(s), C_(n) ^(d) arecoefficients obtained during training processes based upon clinicaldata.

In particular, blood pressure monitoring system 20 calculates a systolicblood pressure value (S), a mean blood pressure value (M), a diastolicblood pressure value (D) based upon the following formulas:

    M=C.sub.1.sup.m P.sub.1.sup.m +C.sub.2.sup.m P.sub.2.sup.m +. . . +C.sub.n.sup.m P.sub.n.sup.m

    S=C.sub.1.sup.s P.sub.1.sup.s +C.sub.2.sup.s P.sub.2.sup.s +. . . +C.sub.n.sup.s P.sub.n.sup.s

    D=C.sub.1.sup.d P.sub.1.sup.d +C.sub.2.sup.d P.sub.2.sup.d +. . . +C.sub.n.sup.d P.sub.n.sup.d

wherein P₁ ^(m), P₁ ^(s), P₁ ^(d), . . . , P_(n) ^(m), P_(n) ^(s), P_(n)^(d) are parameters derived from waveform pressure amplitudes. Suchparameters may be calculated from shape characteristics of the waveformor parameters calculated from functions such as curves based uponrelationships between particular points of several waveforms. Theparameters may be further based upon hold down pressure values and timeperiods between particular points on the waveforms. The value C₁ ^(m),C₁ ^(s), C₁ ^(d), . . . , C_(n) ^(m), C_(n) ^(s), C_(n) ^(d) arecoefficients obtained during training processes based upon clinicaldata.

In addition, the pulse rate (PR) may also be determined using theformula: ##EQU1##

To determine pulse rate, four individual waveforms or beats are sensedand are time averaged to determine pulse rate. Preferably, the waveformsused to determine pulse rates include the waveform having largestmaximum pressure amplitude, the two waveforms prior to the waveformhaving the largest maximum pressure and the waveform succeeding thewaveform having the largest maximum pressure. Once the four waveformsare identified, the pulse rate of each waveform is determined. The sumof the pulse rate of the four waveforms is then divided by four tocalculate pulse rate PR. The pulse rate (PR_(N)) for each waveform isbased upon the following formula: ##EQU2##

FIGS. 9, 10 and 11 illustrate representative parameters which may beused to calculate blood pressure values. FIG. 9 illustrates a sampleseries of waveforms exhibited by the underlying artery as a varyingpressure is applied over time. The vertical scale indicates pressure inmmHg while the horizontal scale indicates individual sample points atwhich the blood pressure values exerted by the pulse are measured overtime. In the preferred embodiment, transducer 40 produces continuouselectrical signals representing waveform pressures which are sampled 128times per second.

In the preferred embodiment, the hold down pressure applied by hold downpressure assembly 36 to sensor interface assembly 38 (shown in FIG. 1)is swept over a preselected range of increasing hold down pressures.Preferably, the sweep range of hold down pressures is begun atapproximately 20 mmHg. The hold down pressure applied by hold downpressure assembly 36 is then steadily increased until two individualwaveforms are sensed following the sensed waveform having the largestpressure amplitude. Alternatively, once the waveform having the largestmaximum pressure amplitude is sensed and identified, successive sweepsmay alternatively have a varying hold down pressure applied until apreselected multiple of the mean hold down pressure of the waveformhaving the largest maximum pressure amplitude is reached. Preferably,each sweep range extends between the initial hold down pressure of 20mmHg and a final hold down pressure of approximately 150% of the meanhold down pressure of the waveform having the largest maximum pressureamplitude during the previous sweep. In addition, the sweep range mayalternatively have an initial hold down pressure of approximately 20mmHg to a final hold down pressure having a preselected absolute value.Alternatively, the sweep could start at a high pressure and sweep low.As a safety measure, the pressure within pressure chamber (sensed bytransducer 42) and interface chamber 210 (sensed by transducer 40) arecontinually monitored by monitor 26. If the ratio of the pressureswithin pressure chamber 116 and chamber 210 fall outside of a definedrange of limits, an alarm is signaled.

After each hold down pressure sweep, blood pressure monitoring system 20begins a successive new sweep to calculate new, successive bloodpressure values. As a result, blood pressure monitoring system 20continually measures blood pressure within the underlying artery withoutcausing discomfort to the patient. As can be appreciated, the sweeprange of hold down pressure applied by hold down pressure assembly 36may have various initial and final points. Furthermore, the hold downpressure applied by hold down pressure assembly 36 may alternatively beintermittently varied. For example, the hold down pressure may beincreased or decreased in a step-wise fashion.

Based upon sensed and sampled pressure waveform signals or data producedby transducer 40 and sent to monitor 26 during each sweep of hold downpressures, monitor 26 derives preselected parameters for calculatingblood pressure values from the derived parameters and a stored set ofcoefficients. As indicated in FIG. 9, parameters may be derived directlyfrom the absolute waveform pressures which vary as hold down pressure isvaried over time. Such parameters may be derived from the shape of thewaveforms including a particular waveform's slope, absolute pressure ata selected sample point, a rise time to a selected sample point on awaveform and the hold down pressures corresponding to a particularsample point on a waveform. As can be appreciated, any of a variety ofparameters may be derived from the absolute waveform pressures shown inFIG. 9. Parameters may further be based upon particular points orfunctions of the sample points.

FIG. 10 illustrates an example of how values or parameters of multiplewaveforms 500 shown in FIG. 9 may be used to derive additionalparameters. FIG. 10 shows several data points 510. Each data point 510represents a selected waveform taken from the sweep shown in FIG. 9.Curve 520 is derived by fitting points 510 to a preselected function orrelationship. Parameters such as the peak 530 are then derived fromcurve 520. As can be appreciated, various other parameters such as slopemay also be derived from curve 520. Parameters derived from curve 520are ultimately based upon pressure waveforms 500 shown in FIG. 9 whichare produced from sensed pressure waveform data or signals fromtransducer 40. However, because curve 520 is derived using a pluralityof waveforms 500, parameters derived from curve 520 represent theoverall relationship between the plurality of waveforms 500. In otherwords, parameters derived from curve 520 represent the way in which theplurality of waveforms 500 (shown in FIG. 9) are related to one another.Data points 510 represent corrected, relative waveform pressures. As canbe appreciated, functions such as curves may also be derived usingabsolute waveform pressure values which are shown in FIG. 9.

A waveform is "corrected" by subtracting the hold down pressure from theabsolute pressure of the waveform to produce relative waveform pressures(otherwise known as amplitudes). Correcting a waveform eliminatescharacteristics of the waveform which result from a continuouslyincreasing hold down pressure being applied to the artery during eachwaveform or cardiac cycle.

FIG. 11 further illustrates other parameters which may be derived fromwaveform pressure values as shown in FIG. 9. FIG. 11 illustrateswaveform 600 selected from waveforms 500. Waveform 600 is preferably thewaveform having the largest peak or maximum pressure amplitude.Alternatively, waveform 600 may be any of the waveforms 500 (shown inFIG. 9) such as waveforms immediately preceding or succeeding thewaveform having the largest maximum pressure. As shown in FIG. 11,waveform 600 is corrected such that the beginning point 602 and anending point 604 have the same absolute waveform pressure value. Asfurther shown by FIG. 11, waveform 600 is horizontally and verticallyscaled to eliminate gain from parameters derived from waveform 600.Preferably, waveform 600 is scaled from zero to twenty-one beginning atbeginning point 602 and ending at ending point 604 of waveform 600 onthe horizontal b axis. Preferably, waveform 600 is vertically scaledfrom zero to one beginning at its base and ending at its peak. Becausewaveform 600 is horizontally and vertically scaled, parameters may bederived from waveform 600 for calculating blood pressure values withoutthe gain of the particular patient affecting the calculated bloodpressure value. Gains are caused by the differences between the actualpressure exerted within the artery and the pressures sensed at thesurface of the wrist or anatomy which is caused by varyingcharacteristics of the intermediate tissue. Scaling waveform 600eliminates any gains exhibited by individual patients. By using scaledvalues to locate corresponding points or waveform pressure amplitudes onwaveform 600, points on waveform 600 uniformly correspond to the samepoints on waveforms exhibited by other patients.

As shown by FIG. 11, various parameters may be derived from scaled,corrected waveform 600. As shown by FIG. 11, such parameters includewidths of waveform 600 at selected points along the vertical y axis,ratios of individual waveform pressure amplitudes at selected pointsalong the horizontal b axis and the amplitude of the waveform, the risetime or time elapsed from the start of waveform 600 at point 602 to aselected point along the vertical y axis. In addition, several otherparameters may also be derived from waveform 600, such as slope andother shape characteristics.

Once the parameters to be used in calculating blood pressure values areselected, coefficients corresponding to each parameter must bedetermined. Coefficients represent the relationship between a particularparameter set and the resulting blood pressure value to be determinedfrom a particular parameter set. Coefficients are initially ascertainedfrom clinical tests upon patients having known blood pressure values.Typically, the known blood pressure value is determined using the A-linemethod which is generally accurate, although difficult to set up,expensive and medically risky. As the blood pressure is determined usingthe A-line or other methods, sensor interface assembly 38 is positionedover the underlying artery of the patient. Hold down pressure assembly36 applies a varying pressure to the artery of the patient having theknown blood pressure value. As discussed above, transducer 40 producessensed pressure waveform signals or data representing arterial pressurewaveforms. Monitor 26 receives the produced sensed pressure waveformdata and derives preselected parameters from the sensed pressurewaveform data. Coefficients are then determined using the derived valuesof the selected parameters and the known blood pressure value. Eachcoefficient corresponding to each selected parameter is a function ofthe known blood pressure values and the derived parameters. Preferably,several patients are clinically tested to ascertain the coefficients.Once obtained, the coefficients are stored for use in non-invasivelycalculating blood pressure values of other patients without thenecessity of using the more time consuming, expensive and risky A-linemethod and without using the generally more inaccurate conventionalblood pressure measuring methods. Each particular coefficient ispreferably ascertained so as to be applicable for calculating bloodpressure values from the derived waveform parameters of all patients.Alternatively, individualized coefficients may be used to calculateblood pressure values from derived waveform parameters of particularpatients falling within a particular age group or other specializedgroups. The coefficients are preferably determined for use with the sameblood pressure monitoring system as will be used to determine theparticular blood pressure value of patients having unknown bloodpressure values. However, as can be appreciated, the method of thepresent invention for ascertaining coefficients as well as the method ofthe present invention for determining blood pressure values may be usedin conjunction with any one of a variety of blood pressure monitoringsystems including different sensor assemblies and hold down pressureassemblies.

In addition to illustrating various methods by which parameters may bederived from waveform pressure data, FIGS. 9, 10 and 11 illustrateparticular parameters for use in calculating a systolic, a mean and adiastolic blood pressure value of a particular patient during anindividual hold down pressure sweep. According to the preferred methodof the present invention, hold down pressure assembly 36 applies asweeping, continuously varying hold down pressure to the underlyingartery. Preferably, the hold down pressure applied by hold down pressureassembly 36 during each sweep begins at 20 mmHg and ramps upward overtime until at least two waveforms are detected by transducer 40 afterthe waveform having the largest maximum pressure is identified. Basedupon the produced sensed pressure waveform data representing thewaveforms as representatively shown by FIG. 9, blood pressure monitoringsystem 20 calculates systolic, mean and diastolic blood pressure using astored set of coefficients. Systolic blood pressure (S) is calculatedusing the formula:

    S=C.sub.1.sup.s P.sub.1.sup.s +C.sub.2.sup.s P.sub.2.sup.s +C.sub.3.sup.s P.sub.3.sup.s +C.sub.4.sup.s P.sub.4.sup.s +C.sub.5.sup.s P.sub.5.sup.s +C.sub.6.sup.s P.sub.6.sup.s +C.sub.7.sup.s P.sub.7.sup.s +C.sub.8.sup.s P.sub.8.sup.s +C.sub.9.sup.s

Coefficients C₁ ^(s) -C₉ ^(s) are stored coefficients ascertainedaccording to the earlier described method of the present invention. C₉^(s) is an offset value. Parameters P₁ ^(s) and P₂ ^(s) are derived fromrelative waveform pressure amplitudes corresponding to scaled valuestaken from a scaled and corrected beat as represented by waveform 600 inFIG. 11. Preferably, parameter P₁ ^(s) is the ratio defined by thewaveform pressure amplitude on waveform 600 which corresponds to scalevalue b₁ along the horizontal axis divided by the maximum waveformpressure amplitude or peak (point 606) of waveform 600. Parameter P₂^(s) preferably is the ratio defined by the waveform pressure amplitudeof point 608 on waveform 600 that corresponds to scale value b₃ alongthe horizontal b axis divided by the maximum waveform pressure amplitudeor peak (point 606) of waveform 600.

Parameter P₃ ^(s) is the rise time or the time elapsed from the start ofthe waveform to a particular point along waveform 600 corresponding to aparticular vertical scale value. Preferably, parameter P₃ ^(s) is theelapsed time from the start of waveform 600 to a point 610 on waveform600 which has a vertical height of approximately 0.18 that of a maximumpressure amplitude or peak (point 606) of waveform 600. This rise timeor elapsed time is represented as 612 in FIG. 11.

Parameter P₄ ^(s) is the mean pressure of the uncorrected waveform 500a(shown in FIG. 9) having the highest peak or maximum pressure. ParameterP₄ ^(s) is indicated on FIG. 9 by point 700. Parameter P₅ ^(s) is thesystolic point of the uncorrected pressure waveform immediatelyfollowing the uncorrected pressure waveform having the largest maximumpressure. Parameter P₅ ^(s) is represented by point 710 on FIG. 9.

Parameter P₆ ^(s) is a parameter taken from a function such as a curvederived from values of a plurality of waveforms 500 (shown in FIG. 9).Preferably, parameter P₆ ^(s) is the peak pressure of curve 520 shown inFIG. 10. The peak is represented by point 530. Curve 520 is preferablygenerated by fitting the relative waveform pressure amplitude ofwaveforms 500 (shown in FIG. 9) to the function or mathematicalexpression of:

    AMPLITUDE=exp(ax.sup.2 +bx+c),

wherein x=the mean pressure amplitude of each pressure waveform.

Parameter P₇ ^(s) is a time value representing a width of waveform 600(represented by segment 614 between points 616 and 618) whichcorresponds to a selected percentage of the maximum pressure amplitudeor peak (point 606) of waveform 600. The time elapsed between points 616and 618 is determined by counting the number of samples taken by monitor26 which lie above points 616 and 618 on waveform 600. Preferably,parameter P₇ ^(s) is the width of waveform 600 at a height of about 0.9A, where A is the maximum waveform pressure amplitude of waveform 600(point 606).

Parameter P₈ ^(s) is the maximum slope of the uncorrected waveform 500cimmediately following the waveform 500a having the largest maximumpressure or peak.

The mean blood pressure value (M) is calculated using the formula:

    M=C.sub.1.sup.m P.sub.1.sup.m +C.sub.2.sup.m P.sub.2.sup.m +C.sub.3.sup.m P.sub.3.sup.m +C.sub.4.sup.m P.sub.4.sup.m +C.sub.5.sup.m

Coefficients C₁ ^(m) -C₅ ^(m) are stored coefficients ascertainedaccording to the earlier described method of the present invention.Coefficient C₅ ^(m) is an offset. Parameters P₁ ^(m) and P₂ ^(m) arederived from relative waveform pressure amplitudes corresponding toscaled values taken from the scaled and corrected beat as represented bywaveform 600 in FIG. 11. Preferably, parameter P₁ ^(m) is the ratiodefined by the waveform pressure (point 620) on waveform 600 whichcorresponds to the scale value b₉ along the horizontal axis divided bythe maximum waveform pressure amplitude or peak (point 606) of waveform600. Similarly, parameter P₂ ^(m) is the ratio defined by the waveformpressure on waveform 600 which corresponds to scale value b₁₃ along thehorizontal axis (point 622) divided by the maximum waveform pressureamplitude or peak (point 606) of waveform 600.

Parameter P₃ ^(m) is identical to parameter C₁ ^(m) P₄ ^(s) used tocalculate systolic blood pressure. Parameter P₄ ^(m) is identical toparameter P₆ ^(s) used to calculate systolic blood pressure.

Diastolic blood pressure values (D) are calculated using the formula:

    D=C.sub.1.sup.d P.sub.1.sup.d +C.sub.2.sup.d P.sub.2.sup.d +C.sub.3.sup.d P.sub.3.sup.d +C.sub.4.sup.d P.sub.4.sup.d +C.sub.5.sup.d P.sub.5.sup.d +C.sub.6.sup.d P.sub.6.sup.d +C.sub.7.sup.d P.sub.7.sup.d +C.sub.8.sup.d

Coefficients C₁ ^(d) -C₈ ^(d) are stored coefficients ascertainedaccording to the earlier described method of the present invention.Coefficient C₈ ^(d) is an offset value. Parameter P₁ ^(d) is derivedfrom relative waveform pressure corresponding to scaled values takenfrom a scaled and corrected beat as represented by waveform 600 in FIG.11. Preferably, parameter P₁ ^(d) is a ratio defined by the waveformpressure amplitude on waveform 600 which corresponds to scale value b₁₂along the horizontal axis (point 624) divided by the maximum waveformpressure amplitude or peak (point 606) of waveform 600.

Parameter P₂ ^(d) is identical to parameter P₃ ^(s) used to calculatethe systolic blood pressure. Preferably, parameter P₃ ^(d) is the widthof segment 626 between points 628 and 630. Preferably points 626 and 628are points along waveform 600 that are located at a height of 0.875 A,where A is the maximum pressure amplitude (point 606) of waveform 600.The width or time of parameter P₃ ^(d) is determined by counting thenumber of individual waveform pressure amplitude signals or samplesgenerated by transducer 40 and transmitted to monitor 26 which lie abovepoints 626 and 628 on waveform 600. If points 626 and 628 fall betweenindividual waveform pressure amplitude signals or samples, interpolationis used to determine the time width of parameter P₃ ^(d).

Parameter P₄ ^(d) is identical to parameter P₄ ^(s) used to calculatesystolic blood pressure. Parameters P₅ ^(d) and P₆ ^(d) are calculatedfrom absolute waveform pressures as illustrated in FIG. 9. Preferably,parameter P₅ ^(d) is the diastolic pressure value of the uncorrectedwaveform having the largest maximum pressure value. This diastolic valueis represented by point 720. Parameter P₆ ^(d) is the diastolic pressurevalue of the uncorrected waveform (waveform 500c) immediately followingthe waveform (waveform 500a) having the largest maximum pressureamplitude or peak. Parameter P₆ ^(d) is represented by point 730 on FIG.9.

Parameter P₇ ^(d) is derived from absolute waveform pressuresillustrated in FIG. 9. To derive parameter P₇ ^(d), the slopes along theportions of each individual waveform 500 are determined. Parameter P₇^(d) is the hold down pressure applied to the underlying artery thatcorresponds to the point on the particular waveform having the maximumslope corrected amplitude. The slope corrected amplitude of a waveformis obtained by multiplying its amplitude with the maximum slope over allwaveforms 500 and dividing the result with the slope corresponding tothe individual waveform. As can be appreciated, various alternativeparameters may also be used to calculate blood pressure values under themethod of the present invention.

VI. Conclusion

The present invention enables blood pressures of patients to becontinuously and non-invasively determined without the complexity, cost,risks, and inaccuracies associated with the prior methods andapparatuses for determining blood pressure. Wrist assembly 24 securelymounts sensor interface assembly 38 upon wrist 22 of the patient so thatpatient movement does not alter the optimal location of sensor interfaceassembly 38 found. The lower pivot point of sensor interface assembly 38causes pressure applied by the sidewall of assembly 38 to the tissueabove the underlying artery to be uniform around the perimeter of thesidewall. As a result, blood pressure monitoring system 20 samples moreaccurate signals representing blood pressure pulse waveforms. Byderiving parameters from the waveform data and using storedcoefficients, blood pressure monitoring system consistently andaccurately determines blood pressure values.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. For example, although the determination ofpressure values based upon waveform parameters has been described usinglinear equations and stored coefficients, other methods using non-linearequations, look-up tables, fuzzy logic and neural networks also can beused in accordance with the present invention.

What is claimed is:
 1. A method of locating a blood pressure sensor overan underlying artery having a pulse, the method comprising:positioningthe blood pressure sensor at each of a plurality of locations above aknown approximate location of the artery while applying hold downpressure at each location; wherein the blood pressure sensor has anactive area which contacts a patient's body and has a width greater thana width of the artery, and wherein the blood pressure sensor produces asingle time-varying signal representative of pressure applied to theactive area by the artery during a cardiac cycle; sensing pressureapplied to the active area by the artery during a cardiac cycle with thesensor at each of the locations to produce at each location thetime-varying signal representative of the pressure applied to the activearea by the artery; and displaying an indication of maximum pressuresignal sensed during each cardiac cycle at each location to aid in finalpositioning of the sensor.
 2. The method of claim 1 wherein displayingan indication includes:deriving waveform pressure data from thetime-varying signal deriving a value from the waveform pressure data ateach of the plurality of locations, the value comprising a maximumpressure amplitude exerted by the pulse during a cardiac cycle inresponse to hold down pressure applied to the artery; and displaying thevalue at each of the plurality of locations.
 3. The method of claim 2wherein displaying the value includes providing a digital output of thevalue.
 4. The method of claim 1 wherein the pulse exerts a pressure uponthe sensor which varies over time during an individual cardiac cycle andwherein displaying an indication includes:visually displaying thepressure exerted by the pulse upon the sensor as a function of timeduring an individual cardiac cycle.
 5. The method of claim 1 wherein thepulse exerts a pressure upon the sensor which varies over time in theshape of a pressure waveform having a systolic portion and a diastolicportion during an individual cardiac cycle, wherein the diastolicportion has a known characteristic shape and wherein the hold downpressure applied to the sensor is as large as possible without the knowncharacteristic shape of the diastolic portion of the waveformdistorting.
 6. The method of claim 1 wherein the step of sensingpressure during a cardiac cycle includes:continually sensing pressureduring a cardiac cycle with the sensor at each of the locations.
 7. Amethod of locating a sensor over an underlying artery having a bloodpressure pulse, the method comprising:positioning the sensor at each oneof a plurality of locations above a known approximate location of theartery while applying hold down pressure to the artery, wherein thesensor has an active area which contacts a patient's body and has awidth greater than a width of the artery, and wherein the sensorproduces a single time-varying signal representative of pressure appliedto the active area by the artery during a cardiac cycle; sensingpressure applied to the active area by the artery during a cardiac cyclewith the sensor at each of the locations to produce at each location thetime-varying signal representative of the pressure applied to the activearea by the artery; deriving waveform pressure data from the timevarying signal; and positioning the sensor over the underlying arterybased upon the sensed waveform pressure data at each one of theplurality of locations.
 8. The method of claim 7 including:providing anindicator based upon the sensed waveform pressure data at each one ofthe plurality of locations to aid in final positioning of the sensor. 9.The method of claim 8 wherein providing an indicator includes:deriving avalue from the waveform pressure data at each of the plurality oflocations; and displaying the value at each of the plurality oflocations.
 10. The method of claim 9 wherein the value comprises amaximum pressure amplitude exerted by the pulse during a cardiac cyclein response to hold down pressure applied to the artery.
 11. The methodof claim 9 wherein displaying the value includes providing a digitaloutput of the value.
 12. The method of claim 8 wherein the pulse exertsa pressure upon the sensor which varies over time during an individualcardiac cycle and wherein providing an indicator includes:visuallydisplaying the pressure exerted by the pulse upon the sensor as afunction of time during an individual cardiac cycle.
 13. The method ofclaim 7 wherein the pulse exerts a pressure upon the sensor which variesover time in the shape of a pressure waveform having a systolic portionand a diastolic portion during an individual cardiac cycle, wherein thediastolic portion has a known characteristic shape and wherein the holddown pressure applied to the sensor is as large as possible without theknown characteristic shape of the diastolic portion of the waveformdistorting.
 14. The method of claim 7 wherein the step of sensingpressure during a cardiac cycle includes:continually sensing pressureduring a cardiac cycle with the sensor at each of the locations.